This invention relates generally to signal processing techniques for fiber optic sensor systems. This invention relates particularly to processing signals indicative of a phase shift in an optical signal in both frequency-division-multiplexed (FDM) and time-division-multiplexed (TDM) sensor systems. Still more particularly, this invention relates to processing signals proportional to the sine and cosine of the phase shift in an optical sensor system to determine the magnitude of the phase shift.
Fiber optic sensor systems of both the FDM and TDM varieties acquire in the demodulation process one term proportional to the sine of the sensor phase shift and another term proportional to the cosine of the phase shift. The sine of the sensor phase shift is referred to as the quadrature term, Q; and the cosine of the sensor phase shift is referred to as the in-phase term, I. The angle of the phase shift is determined by calculating ratio Q/I, which is the tangent of the sensor phase shift. The amplitudes of the sine and cosine terms must be set equal by a normalization procedure to ensure the successful implementation of an arctangent routine to find the sensor phase shift.
Previous normalization methods require observing the phase shift over at least one cycle of 2.pi. radians to obtain peak values of the sine and cosine terms. Preferably the phase shift is observed over many cycles. Unfortunately, there are considerable drawbacks to this requirement.